Method of producing a strain-sensitive resistor

ABSTRACT

A method of producing a strain sensitive resistor includes matching an area of a film insulating layer to a shape of a recess in a surface of a support element. The film insulating area is applied to the support element so that the area of the film matching the shape of the recess is congruent with the shape of the recess. A resistive layer comprising strain-sensitive resistors is applied to the film insulating layer and the film insulating layer and the resistive layer are applied to heat treatment.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Divisional of U.S. patent application Ser. No.09/647,424, filed Nov. 30, 2000, now U.S. Pat. No. 6,512,445, which is a§ 371 of PCT/EP99/02207, filed Mar. 31, 1999, which claims priority tocorresponding application filed in Germany, No. 198 14 261.7, Mar. 31,1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a strain-sensitive resistor comprising aresistance layer arranged on a support element and an electromechanicaltransducer produced with this resistor.

2. Description of the Prior Art

The as yet unpublished German patent application 197 47 001.7 disclosesa strain-sensitive resistor in which the resistor is arranged on a shaftconstructed as a support element. The shaft is subjected to mechanicalloading, the surface strain resulting from this being picked off by thestrain-sensitive resistor arranged on this shaft without anyintermediate support. The resistance layer is applied to the shaft inthe form of a resistive paste, using a printing technique, andintimately connected to said shaft after a heat treatment.

In the event of torsion of the shaft, two main strains at 45° are formedon the surface of the shaft. These strains are evaluated in order todetermine the torque. In this case, the main strains have the samemagnitude but the opposite sign.

The thick-film resistance pastes used have positive strain factors (Kfactors) both for the longitudinal and for the transverse strains, forwhich reason only the difference of these two factors can be used todetermine the effective resistance change. The signal voltagesregistered as the torque sensor are therefore very low and must beamplified by a comprehensive electrical arrangement, as a result ofwhich the influence of interference on the small measured signal isincreased.

SUMMARY OF THE INVENTION

An object of the present invention is to specify a strain-sensitiveresistor in which an increase in the measured signal picked off acrossthe resistor can be implemented in a simple way.

According to the present invention, the object is achieved by a supportelement having a recess on its surface which, when the support elementis subjected to mechanical stress in at least one area of the surface ofthe support element in which the resistance layer is positioned,produces a ratio between the two main strains in the resistance layerwhich differs in magnitude.

The advantage of the invention resides in the fact that the signalresponse of the sensor is increased simply, without any complex changeto the shaft geometry. Such a sensor is suitable for mass production,since it car be produced cost-effectively and quickly.

Because of the recess, the mechanical stresses acting on the supportelement, such as torsion and flexure, are superimposed, the strains inthe main directions (longitudinal, transverse) having unequalmagnitudes.

This recess can be produced by changes which can be implemented simplyon the support element, such as drilled holes, notches and slits, sothat the two main strain directions on the surface of the supportelement on account of torsions no longer have the same magnitudes.

The recess is advantageously formed as a continuous opening in thesupport element.

In one embodiment, the opening is formed as a slot,the resistance layerbeing arranged in the vicinity of the radial area of the slot.

The strain-sensitive resistor can be produced particularly simply if theresistance layer is arranged on a planar surface of the support element.

If the support element of the resistance layer at the some timeconstitutes the component to be stressed mechanically by torsion, it ispossible to dispense with an intermediate support between thestrain-sensitive resistor and the component to be loaded. The mechanicalloading to be detected is in this case picked off directly from thecomponent to be loaded, without signal distortions produced by theintermediate support being produced. Such a resistor reduces theproduction costs considerably.

A reliable, nondetachable connection is achieved if the component to beloaded mechanically and the resistance layer are connected to each othervia an intimate connection, for example are sintered. This is achievedby the paste-like resistance layer applied to the support element by aprinting technique being sintered to the mechanical component during ahigh-temperature process.

In a further embodiment, the support element is electrically conductive,an insulating layer being arranged between the resistance layer and thesupport element. Such a configuration is particularly practical if thecomponent to be loaded mechanically consists of metal, such as is thecase, for example, when it is used a torque sensor in power steeringsystems. In this way, short circuits on the sensor can reliably beprevented.

In a refinement, the insulating layer is paste-like and is applied tothe support element before the application of the resistance layer, in aself-contained high-temperature process, or together with the resistancelayer, during a high-temperature process.

As an alternative to this, the insulating layer, as described, issintered to the support element, either independently or together withthe resistance layer, if the insulating layer is film-like.

In this case, the insulating layer enters into an intimate connectionwith the component to be loaded. This connection can be implemented by areliable process and is extremely stable in the long term.

In particular, the production of the strain-sensitive resistance with afilm-like insulation layer permits the application of the strain gage toa component with a nonplanar surface.

In a method of producing the strain-sensitive resistor in which theinsulating layer and the resistance layer are applied to the supportelement one after another, the film-like insulating layer bearing theresistance layer in at least one area is approximately matched to theshape of the recess in the surface of the support element, this areabeing applied to the surface of the support element so as to conformwith this shape of the recess and subsequently being subjected to theaction of heat.

This has the advantage that the insulating layer is used as anadjustment aid at the same time during the subsequent application of theresistor structure to the support element, and it is therefore ensuredthat the resistance layer is arranged in that area of the supportelement where the greatest difference occurs between the longitudinaland transverse strain.

In a further embodiment of the invention, the insulating layer and/orthe resistance layer are first arranged on a support sheet. The side ofthe support sheet that bears the insulating and/or resistance layer isthen covered with a flexible film layer, whose adhesion to theinsulating layer and/or resistance layer is greater than the adhesion ofthe support sheet to the insulating layer and/or resistance layer. At,least one area of the film layer being matched to the shape of therecess in the surface of the support element. The film layer with theinsulating layer and/or resistance layer is applied to the supportelement in such a way that the correspondingly shaped areas of therecess and the film layer are aligned congruently, the support elementsubsequently being subjected to the action of heat to burn out the filmlayer and sinter on the insulating layer and/or resistance layer.

The advantage of the invention resides in the fact that the existingstructure is produced on a support material in the form of the supportsheet and, after production, is placed on the support element with theaid of the transport film in the manner of a transfer. In this case,too, the shape of the transport film makes the adjustment of thisarrangement on the support element easier. On the basis of thisprocedure, the desired layer structure can be transferred onto anyconceivable geometric shape of the support element and can be sinteredto form a firmly adhering layer during the subsequent heat treatment.

In this design, both the insulating layer and the resistance layer arearranged on a single support sheet and are transported with only onefilm layer.

In this way, a resistor is produced which adheres reliably to nonplanarsurfaces of support elements, even under long-lasting mechanical andthermal loading. This is advantageous in particular when the supportelement, is a component to be loaded mechanically, to which layers areapplied by sintering.

The insulating layer and/or the resistance layer are advantageouslyapplied to the support sheet by a printing technique and dried. It istherefore possible not only to apply simple unstructured layers but alsostructured structures such as entire resistance networks to a nonplanarsurface of a support element.

Using such a production method, it is possible to produce rollingstructures which have dimensions determined by a computer and which aregiven their necessary geometric structure and dimensions only whenapplied to the nonplanar surface.

In one configuration, the insulating layer is printed onto the supportsheet in the form of a glass frit. After the glass frit has been dried,a conductive paste is applied to the insulating layer as the resistancelayer and dried, the film layer then being applied in the form of asynthetic resin film.

As an alternative to this, an insulating layer arranged on a firstcarrier sheet and dried is applied to the support element by means ofthe film layer and subjected to the action of heat. The resistancelayer, printed onto a second support sheet and dried, is then positionedon the already heat-treated insulating layer with the aid of a secondfilm layer arranged on said resistance layer and is then likewiseheat-treated,

The method has the advantage that, depending on the application, boththe entire structure can be produced on a support sheet and, by means ofa single film layer, can be transported from the support sheet to thesupport element, or else each layer of the structure is producedindividually on a support sheet. The individually produced layer islikewise positioned on the support element by means of a film.

In another embodiment of the invention, an electromechanical transducerhas a device with strain-sensitive resistors which comprises aresistance layer arranged on a common support element. The resistancelayer and the support element are separated by an insulating layer andit is possible for an electrical signal corresponding to the strain tobe picked up across the resistors. In this case, evaluation electronicsfor the electrical signal corresponding to the strain are arranged onthe support element. The support element additionally has a recess onits surface which, when the support element is subjected to mechanicalstress in at least one area of the surface of the support element, inwhich at least one strain-sensitive resistor is positioned, produces aratio between longitudinal and transverse strain which differs inmagnitude.

The invention has the advantage that both the sensor element and thesensor electronics are applied directly to the component to be loadedmechanically.

In a refined embodiment, the strain gages and the structure of theevaluation electronics, such as conductor tracks, contact points,thick-film resistors, are arranged on a common, film-like insulatinglayer, which is then centered jointly onto the component to be loadedmechanically.

This production of sensor element and sensor electronics even permitsarrangement on components which do not have a planar surface, forexample on round components.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention permits numerous embodiments. One of these is to beexplained in more detail using figures illustrated in the drawing and inwhich:

FIG. 1 is a schematic diagram of a torque sensor according to thepresent invention.

FIG. 2 is a sectional view of a strain gage according to the presentinvention along line A-A′ in FIG. 1;

FIG. 3 a is a plan view of a first support element configured accordingto the present invention;

FIG. 3 b is a cross section of the support element along the lineIIIB—IIIB in FIG. 3 a;

FIG. 4 is a plan view of a second support element configured accordingto the present invention;

FIG. 5 is a cross sectional view of a strain-sensitive measurementresistor arranged on a support sheet;

FIG. 6 is a schematic view of another embodiment of the torque sensoraccording to the present invention; and

FIG. 7 is a sectional view of the torque sensor along line VII—VII inFIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Identical features are identified by identical reference symbols.

FIG. 1 shows the strain-gage torque sensor 20 according to the inventionin schematic form. The sensor has a resistance measuring badge whichcomprises tour identically conducted strain-sensitive resistors 4interlinked electrically by conductor tracks 3. The entire extent ofthis resistance bridge is arranged on a dielectric 2, which in turnrests directly on a support 1.

FIG. 2 shows a section through one of the strain-sensitive resistors 4.A dielectric 2 is applied to a support 1, which is cube-like andconsists of steel. Arranged on the dielectric 2 is a conductor track 3having contact areas 5 to connect the resistor 4 electrically to othercircuit parts. The electric resister 4 is in turn arranged son theconductor track 3. The termination is provided by a passivation layer 6,which leaves only the contact areas 5 uncovered. In this case, thesupport 1 is a shaft, on which the surface strain arising frommechanical loading is picked off the shaft directly by means of thestrain sensitive resistors 4. The strain gage described is produceddirectly on the support 1 using thick-layer technology, and in thepresent case the support 1 is identical with the component stressed bytorsion, for example a shaft.

FIG. 3 a shows a plan view of a rectangular surface 9 of the shaft 1.Centrally along its longitudinal extent, the shaft 1 has a slot 7 whichpasses completely through the shaft 1. At its ends, the slot 7 hasradial areas 8 in which, in the event of a torsion acting on the shaft1, two main stresses of unequal magnitude occur on the surface 9 of theshaft 1 along the mid-line M illustrated, and from the point of view ofthe resistor 4 these correspond to a longitudinal strain L and atransverse strain T.

This effect is utilized in order to generate a signal which is picked upat the contact areas 5 from the bridge comprising the four strain gages(see FIG. 1).

The common dielectric 2 for all the measuring resistors 4 is matched tothe radial area 8 of the slot 7.

In the case of a shaft having a rectangular cross section without anygeometrical changes, the two main stresses at 45° to the shaft on thesurface 9 are utilized. The thick-layer resistance pastes have positiveK factors for the longitudinal and transverse strain. The effectiveresistance change is determined from the difference between the Kfactors:$\frac{\Delta\quad R}{R} = {{{ɛ_{L} \cdot K_{L}} + {{ɛ_{T} \cdot K_{T}}\quad\text{where}\quad ɛ_{L}}} = {- ɛ_{T}}}$

This results in K_(Effective)=ε·(K_(L)−K_(T))

(Here, K_(L) is the K factor for the longitudinal strain.

-   -   K_(T) is the K factor for the transverse strain    -   ε is the strain

In the rectangular shaft 1 described (FIG. 3 b), because of the slot 7,the strain gage is arranged in the way described in the radial area 8 ofthe slot 7. As a result, the mechanical stresses acting on the surface,for example torsion and flexure, are superimposed. The resulting mainstrains in this case do not have the same magnitude but a ratio betweenlongitudinal and transverse strain of, for example, 1: −0.3.K _(Effective)=ε·(Y·K _(L) −X·K _(T)), where X=0.3 and Y=1.

FIG. 4 shows an embodiment of the shaft 1′ in which a semicircularrecess 7′ is made in each longitudinal edge. The resistors 4 arearranged in the radial areas 8 of this semicircular recess 7′, asexplained in connection with FIG. 3. A section along the mid-line M, inwhich the two opposite semicircular recesses 7′ are at the smallestdistance from each other, remains free of resistors.

Both the shaft 1 illustrated in FIG. 3 and the shaft 1′ illustrated inFIG. 4 permit the redundant arrangement of two resistance areas, forexample one resistance bridge in each case in each radial area 8 of theslot 7.

On the basis of this simple mechanical arrangement, a signal increaseover the prior art arrangement of up to more than 300% can be achieved,depending on the thick-layer resistance paste used.

In order to produce an intimate connection between the dielectric 2 andthe support 1, in a first design the dielectric 2 is applied to theshaft 1 by means of a nonconductive paste using a printing technique. Inthis case, the paste contains a glass frit which can be sintered at alower temperature than the material of the shaft 1. After the paste hasbeen applied, a conductive layer is applied, likewise using ascreen-printing technique, and forms the conductor track 3 and thecontact areas 5, on which in turn the resistance layer forming theresistors 4 is arranged.

The shaft 1 prepared in this way is heat-treated in a high-temperatureprocess at a temperature of approximately 750° to 900° C. In theprocess, the glass layer is sintered to the surface of the steel of theshaft 1. During this sintering-on process, oxide bridges are formedbetween the dielectric 2 and the shaft 1 and ensure a nondetachableconnection between the shaft 1 and dielectric 2.

This rigid, intimate connection produces a lower strain hysteresis ascompared with the adhesive bonding technique.

An alternative to the insulating layer 2 can also be applied as aflexible film layer. In this case, in a first step the conductor track 3and the contact areas 5, as well as the resistors 4, are applied to thefilm-like dielectric 2 in a manner already known. The film-likedielectric is then placed onto the shaft 1. The area 8′ of the film-likeinsulating layer 2 that is matched to the radial area 8 of the slot 7 isin this case used as an adjustment aid, in order that thestrain-sensitive resistors 4 can be arranged in that area of the shaft 1in which the greatest differences occur between the lateral andtransverse strain forces when the shaft 1 is subjected to torsion.

The above-described film-like dielectric 2 comprises a synthetic resinwith a glass frit, on which the pattern of the resistor 4 is applied bymeans of a screen printing technique. In the screen printing technique,the conductor track 3 with the contact areas 5, and the measuringresistor 4 and then the passivation layer 6 are applied one afteranother. Resistor 4 and conductor track 3 are conductive pastes, whichcontain conductive particles and glass frits. During a high-temperatureprocess at about 850° C., all the layers are sintered onto the shaft 1and the plastic contained in the dielectric 2 is gasified without anyresidues. Here, too, the production of oxide bridges between shaft 1 anddielectric 2 produces a durable connection between the two. After thesintering process, the structures of insulating and conductive layersremain on the shaft 1.

FIG. 5 illustrates an arrangement for a strain-sensitive resistor whichis produced separately from the actual support element, i.e., the shaft1, and subsequently applied to this shaft 1.

A nonconductive paste is printed onto a support sheet 10, which maycomprise a commercially available waxed paper, for example, using thescreen printing process and is dried. The paste contains a glass fritwhich can be sintered at a lower temperature than the material of theshaft 1. After the paste has been dried, the conductor track 3 islikewise printed on by screen printing and dried . Then in order toproduce the resistance layer 4, a conductive paste containing platinumparticles is printed onto the conductor-track layer 3. After thisresistance layer 4 has been dried, the entire structure comprisingdielectric 2, conductor track 3 and resistor 4 is completely coveredwith a flexible synthetic resin layer 11, which acts as a film. At itsedges this film it adheres to the support sheet 10.

This prepared arrangement is then removed from the support sheet 10using the principle of a transfer, in that the flexible film layer 11 isused as a transport aid for the dielectric 2, the conductor track 3, andthe resistor 4,

Since the adhesion of the dielectric 2 to the support sheet 10 issignificantly lower than to the flexible film layer 11, when the supportsheet 10 and the flexible film layer 11 are separated, the entireresistor structure and the dielectric 2 always remains on the film layer11.

This film layer 11 is placed onto the shaft 1 in such a way that theradial area of the dielectric 2 is made to coincide with the radial area8 of the slot 7. In the process, the dielectric 2 comes into directcontact with the shaft 1. Since the film layer 11 that projects beyondthe dielectric 2 has adhesive properties, the above-describedarrangement remains in its position applied to the shaft 1.

Before the structure in rig. 5 is applied to the shaft 1, the steel iswetted with an adhesion promoter in order to fix the dielectric 2 on theshaft 1 better.

During the subsequent high-temperature process, the flexible film layer11 is burned or gasified at a temperature of approximately 300° C. Whenthe temperature is further increased to approximately 700° to 900° C.,the glass layer of the dielectric 2 is sintered to the surface of theshaft 1. During this sintering-on process, oxide bridges are formedbetween the dielectric 2 and the shaft 1 and ensure a direct connectionbetween shaft 1 and dielectric 2.

After the flexible film layer 11 has been gasified without any residues,the dielectric 2, the conductor tracks 3, and the resistors 4 remain onthe shaft 1 in the form of insulating, conductor-track and resistancelayers.

As a result of the support sheet 10, the arrangement is very practicalto handle, since there is no risk of inadvertent bonding of the flexiblefilm layer 11 before the support sheet 10 is pulled off.

FIG. 6 shows a thick-layer torque sensor 21 having the strain gages justexplained, which is used in auxiliary-force devices in motor vehicles,in particular in electrical or electrohydraulic power steering systems.

The shaft 1 to be loaded has a parallel piped configuration. Arranged onthe shaft 1, in the manner described above, is a dielectric 2 on which aresistance measuring bridge 12 is applied by means of the measuringresistors 4 acting as strain gages. The resistance measuring bridge 12comprises, in a known manner, four resistors 4 which are connected viaconductor tracks 3 to electric contact areas 5.

As can be seen from FIG. 6, the resistance measuring bridge 12 which isapplied by a thick-layer technique, is connected to the evaluationelectronics 14 via a conductor track 13, likewise produced by athick-layer technique. The evaluation electronics 11 comprise discretecomponents 16, which are connected to the resistance measuring bridge 12at the contact areas 5. These evaluation electronics 14 can be arrangedseparately or else, as in the present case, directly on the shaft 1,where they are soldered to the contact areas 5.

In order to transmit the sensor signal without contact, a coil 15,likewise made of a conductor track produced in a thick-layer techniqueand contact areas, is formed on the shaft 1 and connected to theevaluation electronics 14. Alternatively, the coil can also beconstructed using a conventional technique (winding).

The possibility of printing the coil 15 on using a thick-layer techniquemeans that external soldered connections can be dispensed with. In thiscase, contact is advantageously made with the evaluation electronics 14at the contact areas 5 by means of a surface-mounted device technique.This produces a an embodiment, which comprises the sensor element andthe electronics and can be set up directly on the shaft 1. Such a sensorcan be potted with plastic, for example silicone.

By means of such a sensor, measuring the surface strain on the shaft 1,in the case of use in power steering systems, direct drive from thewheel to the drive is ensured, without additional elasticity in thesteering shaft.

FIG. 7 illustrates the torque sensor embodiment according to theinvention in section. Discrete components 15 from the evaluation circuit14 are soldered onto the contact areas 5 not covered by the passivationlayer 6.

1. A method of producing a strain-sensitive resistor arrangement,comprising the steps of: (a) applying an insulating layer to the firstside of the support sheet and applying a resistive layer onto theinsulating layer after the insulating layer has been applied to saidsupport sheet, wherein the insulating layer is printed onto the supportelement in the form of a glass frit, the glass frit is dried, and aconductive paste is applied onto the insulating layer as the resistivelayer; (b) matching an area of a flexible film layer to a shape of arecess defined in a surface of a support element; (c) covering the firstside of the first support sheet, the insulating layer, and the resistivelayer with the flexible film layer such that the flexible film coversthe insulating layer and the resistive layer, wherein an adhesion of theflexible film layer to the at least one of the insulating layer and theresistive layer is greater than an adhesion of the first support sheetto the at least one of the insulating layer and the resistive layer; (d)removing the support sheet from the insulating layer and the resistivelayer such that at least the resistive layer remains on the flexiblefilm layer and applying the flexible film layer with at least theresistive layer to the surface of the support element so that the areaof the flexible film layer matching the shape of the recess defined inthe surface is congruent with the shape of the recess defined in thesurface; and (e) subjecting the support element to a heat treatmentafter said step (d) to burn off the flexible film layer and sinter onthe at least one of the insulating layer and the resistive layer.
 2. Amethod of producing a strain-sensitive resistor arrangement, comprisingthe steps of: (a) printing an insulating layer on a first side of afirst support sheet and printing a resistive layer on a first side of asecond support sheet; (b) matching an area of first and second flexiblefilm layers to a shape of a recess defined in a surface of a supportelement; (c) covering the first side of the first support sheet and theinsulating layer with the first flexible film layer, wherein an adhesionof the flexible film layer to the insulating layer is greater than anadhesion of the first support sheet to the insulating layer and coveringthe first side of the second support sheet and the resistive layer withthe second flexible film layer, wherein an adhesion of the flexible filmlayer to the resistive layer is greater than an adhesion of the secondsupport sheet to the resistive layer; (d) removing the first supportsheet from the insulating layer such that the insulating layer remainson the flexible film layer and applying the flexible film layer with theinsulating layer to the surface of the support element so that the areaof the flexible film layer matching the shape of the recess defined inthe surface is congruent with the shape of the recess defined in thesurface and so that the insulating layer is applied to the supportelement; and (e) subjecting the support element to heat treatment afterstep (d) to burn off the first flexible film layer and sinter on theinsulating layer; (f) removing the second support sheet from theresistive layer such that the resistive layer remains on the secondflexible film layer and arranging the resistive layer on the insulatinglayer after said step (e); and (g) heat-treating the resistive layer forsintering the resistive layer on the insulating layer.